Resonant ion-dip infrared spectroscopy of benzene-(methanol)(m) clusters with m=1-6

Resonant ion-dip infrared spectroscopy has been employed to record cluster-size-specific spectra of C6H6-(CH3OH)(m) with m=1-6 in the OH stretch fundamental region. The comparison of the spectra with the results of ab initio calculations on the pure methanol clusters enables the assignment of the hydrogen-bonding architecture in the clusters. In all cases, the methanol molecules aggregate together in a single subcluster. With m=1, a single infrared transition is observed, redshifted from that of a free methanol momomer by 42 cm(-1) due to rr hydrogen bonding between benzene and methanol. The m=2 spectrum features two strong transitions at 3506 and 3605 cm(-1). The lower frequency peak is redshifted from the free monomer value by 175 cm(-1) and is assigned to the proton donor in the methanol dimer subcluster. The proton acceptor, which would be a free OH stretch in the absence of benzene, is redshifted by 76 cm(-1) due to a strengthened pi hydrogen bond. In benzene-(CH3OH)(3), three sharp OH stretch transitions are observed at 3389, 3435, and 3589 cm(-1). The comparison of these absorptions with ab initio calculations and with experiments on the pure methanol trimer leads to a structure for benzene-(CH3OH)(3) which incorporates a pi hydrogen-bonded methanol trimer chain, confirming the earlier assignment based on its ultraviolet spectrum. The 3589 cm(-1) transition, due to the pi hydrogen bond of the terminal methanol, is redshifted from the free monomer by 93 cm(-1), a value approaching that of the donor methanol in methanol dimer (-107 cm(-1)). The lower frequency transitions in the m=3 spectrum arise from the donor-acceptor and donor OH stretches in the methanol trimer chain. The spectral characteristics change when m=4. The OH stretch transitions are all located in a region around 3320cm(-1) and are significantly broadened compared to the smaller clusters. By comparison with ab initio calculations, the methanol tetramer structure in benzene-(CH3OH)(4) is deduced to be a cyclic methanol tetramer. The spectra for m=5 and 6 are slightly redshifted but similar to m=4 and point toward cyclic structures as well. (C) 1997 American Institute of Physics.